The invention relates to a process for measuring the opacity in gases, especially in exhaust gases or in the atmosphere, at or near the maximum of eye sensitivity in the green wavelength range with a central wavelength range between 550 and 570 nm, as well as a device for measuring the opacity in gases, especially in exhaust gases or in the atmosphere, which device is equipped with an optical filter arrangement for the visible green spectral range at or near the maximum of eye sensitivity within a wavelength range from 550 to 570 nm in the beam path in front of at least one optical detector, and which is connectable with or is provided with an evaluating electronic system.
In opacity meters, at present, according to law or standards, measuring is done with a wavelength in the xe2x80x9cgreen spectral rangexe2x80x9d, at a peak wavelength of 550 to 570 nm and with a cutoff of less than 4% of the peak-value sensitivity of the peak wavelength for transmissions below 420 nm and above 680 nm. The opacity is defined there as measurement of the xe2x80x9cclouding in the visible spectral range of the human eye sensitivityxe2x80x9d. The measurement in this spectral range of the xe2x80x9ceye sensitivityxe2x80x9d is knowingly so chosen that thereby the clouding of the atmosphere or xe2x80x9csmog formationxe2x80x9d, caused by emissions should be checked. Alternatively, very often also the xe2x80x9ck-valuexe2x80x9d in this spectral range is used as measure for the clouding, in which case the two values are reconvertible into one another by the Lambert Beer law:
100-opacity=100*EXP(-k*L), with L=measuring cell length, or measuring path length.
Mainly the xe2x80x9cblackxe2x80x9d soot particles are/were caught by the opacity measurement and at present, in the legislation and the legally prescribed tests, it is assumed that the clouding or the k-value in the green spectral range is caused only by soot particles. In the opacity meters in use at present, with the measurement of the opacity in the xe2x80x9cvisible spectral rangexe2x80x9d at only one defined wavelength, it is not possible to distinguish whether the measurement value xe2x80x9copacityxe2x80x9d or k-value (in mxe2x88x921) is really caused by soot, or not also by other exhaust gas components.
In motors, however, in reality there can occur also potential exhaust gas components (for example some nitrogen compounds, especially NO2) which likewise absorb in this spectral range, and that can bring about a clouding. These additional components are wrongly included as xe2x80x9csootxe2x80x9d in measurements made with the conventional opacity meter systems. In motor designs which were used in earlier years the dominant constituent causing the opacity really was the soot emission; this, however, is no longer valid for the present and future generations of motors.
In modem motor designs, for example those with CRT (Continuous Regenerating Type) exhaust gas treatment, soot particles are largely catalytically oxidized; on the other hand, however, a part of the NO concentrations present in the exhaust gas is transformed into NO2 by these catalytic processes. NO2, however, is a gas component which is likewise absorbed in the green opacity meter spectral range, and is concurrently measured as xe2x80x9csootxe2x80x9d. On the other hand, xe2x80x9cwhitexe2x80x9d, nonabsorbing particles can also occur (for example sulfates with agglomerated water or also other particle-form reacting products such as condensing hydrocarbons), which by weakening of the light in consequence of a light scattering can likewise have an effect on the measuring.
Likewise with the measuring apparatuses which measure in the middle IR-range, such a discrimination cannot be carried out, especially not for sulfates and for NO2. It is not possible to measure NO2 in motor exhaust gases by means of IR absorption through cross-sensitivity with the steam that is present in the exhaust gas, and neither can the water be removed for this measurement by means of a gas cooling, since NO2 that is soluble in water is simultaneously removed along with it. The measurement of the NO2 concentration can occur at present only by chemo-luminescnce detectors (CLD), and there, however, also only indirectly by means of a difference measurement (NOxxe2x88x92NO=NO2).
For similar reasons sulfate particles in the IR range likewise cannot be measured; especially a direct measurement of the opacity constituent resulting from the light scattering of the sulfate particles is not possible in the visible spectral range. The same holds also for the opacity caused by non-absorbing but condensed, and therewith likewise light-scattering, HC particle constituents.
All concepts in effect at present in the IR range for the total particle measurement are based on measurements of the HC total concentrations (as gas or as gas+particles) and on back-reckoning models (thus also are the examples in EP 0 094 374 and EP 0 123 458). Some of the concepts at present on hand for the HC particle calculation are based on complicated measurements at different temperatures, on the filtering of the gas, on measurement of the xe2x80x9cgaseous xe2x80x9d HC concentration present, and on a back calculation of the particle constituents, as represented in EP 0 616 205.
A direct measurement of the xe2x80x9ccloudingxe2x80x9d by the light scattering, which is still definitive for the visible green spectral range is, for physical reasons, not possible in the IR range, since through the proportionality of the effect to the 4th power of the ratio of light wave length to particle size, factually no light scattering for particles from motor exhaust gases is present in the IR range.
A back calculation such as theoretically might be possible at least in measurements of the total absorption spectra of the exhaust gas in IR (NIR to FIR),with rapid and high-resolving FTIR systems which, however, are extremely costly and expensive, even for NO2, ultimately fails on the fact that the momentary dynamic relations, which occur in the free acceleration and that certainly definitively influence the momentary particle composition, cannot be recalculated from the data obtained. The same holds also for measurements with laser diodes, such as are described, say, in EP 0 920 285, in which there, too, only soot and HC are measured.
Because of the great differences among the xe2x80x9cmiddle IRxe2x80x9d wavelength ranges, all of the present-day methods are completely incapable to describe, or can describe only very incompletely, the conditions present in the visible spectral range. In DE 25 57 268 there is described a process for extinction measurement which can be used, for example, for the determination of the smoke density in smokestacks, but also for the measurement of the dust concentration in work-places, of the emission in the lime works environment, as well as for the determination of the visibility range in fog on highways and at airports. There, by extinction measurements at two different wavelengths, a distinction can be made possible between absorbing and non-absorbing particles, primarily between soot or aerosol particles and vapor-form water. It is not determined, however, in what manner and to what extent an opacity in a certain wavelength range affects the value for the opacity in another wavelength range.
The problem of the present invention, therefore, was to find a process which in a simple manner, and avoiding the above-described disadvantages of the state of the art, makes it possible separately to determine the components which are responsible for the clouding in the visible wavelength range and which, for the measurement of opacity on the basis of soot particles, permits a correction by consideration of further components having an effect in the visible range. A further problem was a device for the execution of the process.
For the solution of the above problem the process mentioned at the outset is characterized in that the opacity, additionally, is also measured in at least one second wavelength range which is located in the spectral range between 200 nm to 2xcexc, and which at best slightly overlaps the first wavelength range. The invention is based on the principle that it was surprisingly ascertained that through the use of at least one additional color filter and therewith measurement in a further wavelength range it is possible to distinguish what share of the measured opacity (or of the k-value) is caused by the soot particles, for example, and what share is caused by other components, for example by scattered light of extremely small, non-absorbing particles and/or by other light-absorbing gas components such as NO2.
According to a further feature of the invention it is provided that the measurement values of at least one additionally used wavelength range are automatically compared with the measurement value of the green wavelength range, and that from these a correction is calculated for the measurement value in the green wavelength range.
Advantageously also, the signals of all the wavelength ranges used can be automatically compared with one another, and from this there is automatically determined the signal share of at least one further component that contributes to the opacity in the green wavelength range.
If, according to a further feature of the invention, at least one additional measurement is taken in a range with a central wavelength between 300 and 450 nm, it is possible to ascertain or to calculate the share of the measured opacity (or of the k-value) which is caused in the green light spectral range by xe2x80x9cwhitexe2x80x9d scattering particles. This is possible since it was recognized that for the k-value the effects of the light scattering on small particles are proportional to the 4th power of the light wavelength, while the effect of the soot absorption is linearly dependent on the wavelength of the filter.
If, alternatively or additionally to this there still is performed an additional measurement in a range the central wavelength of which ranges between 600 nm and 2xcexc, there can be determined therewith the contribution of most gas components which likewise have a measuring effect on the green spectral range, and the signal share caused by the scattered light can be determined, so that these shares can be taken into account for the correction of the measurement value in the visible range, and the contribution of the xe2x80x9csootxe2x80x9d can be determined appreciably more accurately.
Preferably, the central wavelength of the additional measurement, there, lies in the range between 600 nm and 1.2xcexc.
According to a further feature of the invention it is provided that the measurements in the various wavelength ranges are executed automatically controlled in succession, so that a manual intervention is avoided and the measuring series is feasible rapidly and simply.
Still more rapidly, even though with somewhat higher expenditure in apparatus, the process of the invention can be carried out according to one of the preceding paragraphs when the measurements are simultaneously carried out in all wavelength ranges.
The device described at the outset for the execution of the process of the invention is characterized for the solution of the problem posed in that additionally at least one second optical filtering arrangement is provided for a second wavelength range the central wavelength of which is located in the spectral range between 200 nm to 2xcexc, and which overlaps at best slightly with the first wavelength range.
Advantageously the device according to the invention is characterized in that the evaluating electronic element is provided with a circuit or a program which automatically asks for the measurement values, and from them automatically calculates a correction for the measurement value in the green wavelength range.
According to a further inventive feature the evaluating electronic element can be provided with a circuit or a program which asks for the measurement values in all the wavelength ranges used and from them automatically calculates values in order to distinguish the signal shares of at least one component responsible for the opacity in the green wavelength range, from other components likewise absorbing in this wavelength range.
In order to determine the share of the measured opacity (or of the k-value) which is brought about by xe2x80x9cwhitexe2x80x9d scattering particles, according to a further feature of the invention there is provided at least one additional optical filtering arrangement for a wavelength range the central wavelength of which falls in the range between 300 and 450 nm.
On the other hand, also additionally, or alternatively, to the feature just mentioned, at least one additional optical filtering arrangement can be provided for a wavelength range the central wavelength of which falls in the range between 600 nm, and 2xcexc, whereby then the contribution of most gas components in the green spectral range and also the contribution of the scatter light in the green spectral range can be determined and used for the correction of the measurement value on the contribution brought about by the soot.
Preferably it is provided there that the additional optical filter arrangement is for a wavelength range the central wavelength of which falls between 600 nm and 1.2xcexc.
A manual operation is avoided if pushers, swinging arms, turnable disks or the like that carry the optical filter arrangements for the gliding-in or swinging-in of the, or of all optical filter arrangements are provided in the beam path, in front of the detector. Therewith the measuring process is entirely automatable and also more is simply and rapidly feasible than by hand.
Advantageously for a compact and simple construction it can be provided that a blind with openings movable in front of the detector is provided, in which openings the optical filter arrangements are installed.
There preferably for the fully automatic operation of the device there is provided a drive arrangement for the blind and this is connected with the evaluating electronic element. On the other hand, according to a further feature of the invention it can be provided that several detectors are provided parallel and are connected in common with an evaluating electronic element, in which case in front of each detector in the beam path there is provided in each case an optical filter element. Therewith the measurement is more rapidly practicable, so that also rapidly changing relations can be followed in real time.
In the following specification the invention is to be further explained with the aid of a preferred example of execution for the soot particle measurement.
It was possible to establish that especially with modem motor designs certainly a definite share of the xe2x80x9copacityxe2x80x9d can come about by other components besides soot particles, namely especially by NO2, but also in part by xe2x80x9ctransparentxe2x80x9d non-absorbing particles, such as sulfates and water deposited on these sulfate particles.
It was ascertained, surprisingly, that through the use of at least one further xe2x80x9coptical filterxe2x80x9d in the visible spectral range or also in the near infrared up to maximally ca. 2xcexc, the shares of the measuring value in the green spectral range caused by these components, i.e., the total measured opacity, can be determined and therewith the xe2x80x9copacityxe2x80x9d measured in the green spectral range can be selectively separated into the shares which are brought about by NO2 and by xe2x80x9csulfates or also other condensed HC-particlesxe2x80x9d.
There it is of special significance that through this inventive type of measuring in the visiblexe2x80x94not in the middle infraredxe2x80x94spectral range for the first time there exists the possibility of selecting the component NO2 an the share of opacity (or of the k-value) caused by the clouding of NO2 in the green xe2x80x9copacity spectral rangexe2x80x9d, and also additionally to measure the contributions of sulfate particles (and also of the non-absorbing HC particles) in this spectral range. Therewith, however, for the first time it is furthermore directly possible to measure the concentration of NO2 when the measuring system is calibrated by means of an NO2 calibrating gas.
Further, the new invention offers the possibility, also for future exhaust gas tests, separately and highly dynamically to check the share of the soot component and simultaneously the share of NO2 emitted in the free acceleration of motors. In normal idling-measurements or in measurements made with the presently available opacity-meters, the NO2 ejection cannot be measured. The testing of vehicles or motor is therefore substantially simpler to carry out with the process of the invention and the corresponding device as well as of the method used for the evaluation.
A further not-to-be-neglected advantage of this invention is also to be seen in that the concept it uses, in comparison to other measuring concepts such as FTIR or MID NDIR, or laser-diodes measurements, is substantially more economical and simpler to implement and, in addition, the usual sturdy hardware designs can be used for the opacity measuring apparatuses (full current to partial current). The background is that the soot particles absorb strongly and through their usual size distribution the concentration-proportional k-value of the soot particles depends linearly on the light wavelength. On the other hand, most gas components which can affect the measurements in the green spectral range, in the xe2x80x9credxe2x80x9d visible spectral rangexe2x80x9d as well as also near the IR range, no longer absorb, or at least do so substantially less; likewise in the red spectral range the effects through the scattered light become appreciably less or are negligible, so that only xe2x80x9csootxe2x80x9d continues to be measured there and, through following the accepted forms, the shares are separately measurable.
In the event that a substantial share of the measured opacity (or of the k-value) in the green spectral range is caused by xe2x80x9cwhitexe2x80x9d scattering particles, the use of an alternative or additional filter in the xe2x80x9cblue or near UV rangexe2x80x9d likewise permits to discriminate among the effect they cause. The effects brought about by the scattering of light on small particles are proportional to the 4th power of the light wave length, while, as already mentioned above, the effect caused by the soot absorption is linearly dependent on the wavelength of the filter.
The measurement of the different component shares, therefore, can occur by the means that the green filter of the measuring apparatus on the one hand, for example manually by a pusher or also automatically, is replaced by a filter in the blue spectral range (or in the near UV-range). Alternatively also two or even several detectors can be simultaneously equipped with different filters, or the filters can be brought into the beam path in succession, for example by a chopper wheel. Therewith the measurement can occur simultaneously in all wavelength ranges or at least rapidly in one range after another.
In the first case the evaluation must occur either externally after the measuring or also, preferably program-controlled, internally. Altogether, the first-described variant is suited for long-durational constant measurements, while for rapid dynamic measuring processes, the second alternative must be used.
Therewith from the two or three measurement values, by diverse conversion algorithms there can be selectively measured the absorption shares caused by soot or also by other components in the green, standardized spectral range. The concentration, the k-value share or also the contribution to the opacity which is caused by the other absorbing components, such as, for example, NO2 and by light-scattering particles, therefore, can additionally be determined. By this measuring principle and evaluation it is thus possible to distinguished what share of the opacity or of the k-value brought about in the green spectral range is caused by soot, by a gas such as NO2, or by scattering particles such as sulfates or HCs. There can also be calibrated and calculated, furthermore, the concentrations of the gaseous components. From the corrected opacity or k-value for xe2x80x9csootxe2x80x9d there can likewise be calculated or measured its true concentration, since the k-value is directly proportional to the concentration of soot. The determination of the sulfate- or HC-particle shares can occur likewise with this method, at least roughly, there being selectively calculable, here, only the concentrations accumulating as particles. By the variation in the size distribution of the purely scattering particles that are present, the measuring accuracy of the concentration calculation is possible only restrictedly. The measuring or calculating accuracy of the share caused by these particles in the different spectral ranges, however, is not thereby restricted.
With use of three filters in the presence of three components to be considered, or of two filters with the presence of only two components, therewith there is also possible a calculation of the concentration.
In order to avoid possible cross-sensitivities the, additional spectral ranges especially in the near IR range, must be chosen in such manner that no absorptions or only the most minimal absorptions or measuring effects are caused by the water vapor- or CO2-content in the measuring gas.